In recent years, a technique by which a thin film transistor (TFT) is manufactured using a semiconductor thin layer (having a thickness of about several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been applied to a wide range of electronic devices such as ICs or electro-optic devices and urgently developed particularly as switching elements in image display devices. Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and has been used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides exhibit semiconductor characteristics. As examples of metal oxides exhibiting semiconductor characteristics, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like can be given, and thin film transistors in each of which a channel formation region is formed using such a metal oxide exhibiting semiconductor characteristics are already known (see Patent Documents 1 to 5 and Non-Patent Document 1).
Further, not only single-component oxides but also multi-component oxides are known as metal oxides. For example, InGaO3(ZnO)m (m: natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (see Non-Patent Documents 2 to 4).
Further, it has been confirmed that an oxide semiconductor including such an In—Ga—Zn—O-based oxide is applicable to a channel layer of a thin film transistor (see Patent Document 6 and Non-Patent Documents 5 and 6).